As a generality, it can be said that the development of the transistor and the solid state technology which followed the transistor have largely supplanted the use of the vacuum tube. Conventional consumer devices, as well as commercial devices, military devices, and the like, are typically of the solid state variety for a number of reasons. Conventional vacuum tubes are substantially larger than functionally equivalent solid state devices, are inferior in terms of reliability, and consume substantially more power as compared to their solid state counterparts. Furthermore, it has been difficult if not impossible to produce highly functional integrated circuits utilizing vacuum tubes because of their operational and dimensional restrictions. Thus, it can be said that, generally, the vacuum tube device has been superseded by solid state devices.
However, when very high frequency operation is the paramount concern, vacuum tube devices have an advantage over solid state devices. More particularly, at frequencies in the GHz range, delays due to carrier travel become more significant, and the velocity of carriers through the medium of the device becomes increasingly important. It is known that electron travel through semiconductors is slower than electron travel in a vacuum, providing at least a theoretical advantage for the vacuum tube structure, if the path length for carrier travel can be made sufficiently small. Assuming travel distances by carriers can be made comparable, it would be of advantage to provide a structure with carrier travel through a vacuum, rather than through solid state semiconductor material, in order to enhance high frequency response by enhancing carrier transit time.
A disadvantage of vacuum tubes can be avoided if the emitter and collector of the vacuum tube can be located very close to each other. More particularly, by reducing the gap between emitter and collector electrodes to, for example, the order of microns, it becomes possible to emit electrons from a cold cathode (or emitter) by means of electric field emission, eliminating the need for cathode heaters which had been a source of high power dissipation in conventional electron tubes. Cold emitter operation by electric field emission requires not only the very close proximity of the emitter and collector, but also shaping of at least the emitter to enhance the field intensity at a sharp edge on the emitter, thereby locally enhancing the field strength which results in electron emission and travel from emitter to collector.
FIG. 10 shows a cross sectional view of a microminiature vacuum tube of the prior art as described in "A Vacuum Field Effect Transistor Using Silicon Field Emitter Arrays" published in the proceedings of the International Electronic Device meeting 1986 at page 776.
As shown in FIG. 10, a silicon substrate 1 has an insulating film (such as silicon dioxide) formed on the upper surface thereof. Disposed in a gap in the insulating film 2 is a conical cathode 3 (or electron emitter) which is formed by etching the silicon substrate 1. Formed on the surface of the insulating film 2 are an arrangement of anodes 4 (or collector electrodes) and a further intermediate arrangement of control electrodes 5 (hereinafter called gate electrodes). As shown by the dotted line path e.sup.-, electrons emitted from the emitter 3 travel by means of an arcuate path to the collector 4 under the control of voltages imposed on the gate 5.
The microminiature vacuum tube of FIG. 10 represents an attempt to realize certain of the advantages of vacuum tube performance (electron speed) utilizing certain features of microelectronic processing. However, the device is deficient for a number of reasons, one of them being the indirect path for electron travel in which electrons must be emitted from the conical emitter 3 and then flow in an arcuate path about the gate 5 to reach the collector 4.
A further problem results from the fabrication process for forming the device which is not without its difficulties. FIGS. 11a-11d illustrate the prior art process. As shown in FIG. 11a, a silicon substrate 1 has an n-type silicon layer 3a formed on the surface thereof, such as by conventional epitaxial growth processes. The central portion is masked using photolithographic processes to form a centrally disposed metallic etching mask 11 for formation of the conical emitter 3.
As shown in FIG. 11b, the partly completed wafer is then etched by isotropic etching techniques, such as wet etching, conventionally used for silicon, with the metal film 11 as a mask. Because of the isotropic etching techniques, side etching causes the masked portion of the layer 3 to etch more quickly than the portion adjacent the substrate 1, resulting in the illustrated conical shape for the emitter 3.
Having formed the conical emitter, and with the metal film 11 remaining in place, an insulator film 2 is then deposited over the entire surface of the substrate 1, but leaving the central masked portion free of insulator film, as shown in FIG. 11c. The film 11 is then removed and gate electrodes 5 and collector electrodes 4 are formed by conventional sputtering and patterning techniques.
As noted above, problems can arise in following the process sequence illustrated in FIGS. 11a-11d. One of those problems is illustrated in FIG. 12 which shows the variations which can accompany the isotropic etching technique used for forming the conical emitters. More particularly, utilizing the process illustrated in FIGS. 11a-11d, it is quite difficult to reproducibly achieve conical electrodes of the desired size and shape. Because the wet etching process is difficult to control and therefore to reproduce from batch to batch, when a plurality of emitter electrodes 3 are formed on a single silicon substrate, a non-uniformity of the shape of the emitter electrodes is often produced as shown in FIG. 12. It is seen that central conical emitter 3c is of the desired shape and size at the conclusion of etching, whereas emitter 3d represents the over-etched condition in which the emitter electrode is foreshortened, and electrode 3b represents the under-etched condition in which the emitter is not etched to a point. This non-uniformity in etching conditions results in a non-uniformity of characteristics of the devices, which is particularly significant when a plurality of such devices are used in an array of interconnected vacuum tube triodes.
Also as noted above, when a microminiaturized vacuum tube assumes the configuration shown in FIG. 10, electron travel from emitter 3 to collector 4 is in an arcuate path. Because of the arcuate path it is difficult to reduce the distance between the emitter 3 and the collector 4, and that results in a requirement for higher operating voltages. It will be appreciated, of course, that the greater the distance between the emitter and collector in a cold cathode electron discharge device, the greater the operating voltages will be needed to initiate discharge. And utilizing an arcuate path as shown in FIG. 10, and keeping in mind that a gate electrode must be interposed somewhere with respect to an intermediate portion of that path, one will appreciate that the degree to which the device of FIG. 10 can be miniaturized is somewhat limited.